Auto merge of #99780 - Nilstrieb:mir-opt-test-line-no, r=oli-obk

Use line numbers relative to the function in mir-opt tests

As shown in #99770, the line numbers can be a big source of needless and confusing diffs. This PR adds a new flag `-Zmir-pretty-relative-line-numbers` to make them relative to the function declaration, which avoids most needless diffs from attribute changes.

`@JakobDegen` told me that there has been a zulip conversation about disabling line numbers with mixed opinions, so I'd like to get some feedback here, for this hopefully better solution.

r? rust-lang/wg-mir-opt

src/tools/compiletest/src/runtest.rs

@@ -1960,6 +1960,7 @@ impl<'test> TestCx<'test> {
                     "-Zdump-mir=all",
                     "-Zvalidate-mir",
                     "-Zdump-mir-exclude-pass-number",
+                    "-Zmir-pretty-relative-line-numbers=yes",
                 ]);
                 if let Some(pass) = &self.props.mir_unit_test {
                     rustc.args(&["-Zmir-opt-level=0", &format!("-Zmir-enable-passes=+{}", pass)]);